As Illinois Republicans vote in their presidential primary, only one GOP candidate is expected to be in the state. Mitt Romney planned what he hopes to be a victory party Tuesday night in the Chicago suburb of Schaumburg.

Newt Gingrich is campaigning in Louisiana, which votes on Saturday.

Ron Paul is in California, which doesn't vote until June.

And Rick Santorum is in Pennsylvania, his home state, which votes on April 24.

Santorum is planning to watch the Illinois returns and hold what's being called an election night party in Gettysburg. In Illinois, 54 delegates are at stake Tuesday.

But in just over a month, Pennsylvania could be as close to a must-win as any state for the Santorum candidacy.

On April 24, Republicans also vote in four other Northeastern states, where Romney is considered formidable: New York, Connecticut, Delaware and Rhode Island.

A Santorum win in Pennsylvania could be the only way to prevent a five-state Romney sweep. But as Romney has noted at every opportunity, Santorum didn't do so well the last time he faced Pennsylvania voters.

[On] Saturday morning, Santorum will speak at the Pennsylvania Leadership Council conference in Harrisburg, where the state's conservatives will gather to hear from such high-profile politicos as South Carolina Gov. Nikki Haley, former presidential candidate Herman Cain and presidential candidate Newt Gingrich.

Santorum needs to get back in the good graces of the state's fiscal-conservative activists after alienating many with his support for government spending and Arlen Specter.

For Santorum's presidential aspirations, winning Pennsylvania is critical. Not only does it allow him to forge ahead with a battleground state win under his belt, but it also allows him to argue vindication of his crushing defeat in 2006, at least with Republican voters. Santorum, then a two-term incumbent U.S. senator, lost by 18 points to Democrat Bob Casey.